Glaucoma

What is glaucoma?

Glaucoma is the name given to a group of eye conditions which result in damage to the optic nerve at the point at which it leaves the eye (Figure 1.) The optic nerve carries information from the light-sensitive layer in your eye, the retina, to the brain where it is received as a picture. In some people, the damage is caused by raised eye pressure (intraocular pressure or IOP for short). Others may have an eye pressure within normal limits but damage occurs because there is a weakness in the optic nerve. In most cases both factors are involved but to a varying extent.

What controls pressure in the eye?

A layer of cells behind the iris (the coloured part of the eye) produces a watery fluid, called aqueous. Aqueous fluid passes through a hole in the centre of the iris (the pupil) to enter the front chamber of the eye - the anterior chamber. It then leaves the eye through tiny drainage channels in the angle between the front of the eye (the cornea) and the iris, and drains into veins. Normally the fluid produced is balanced by the fluid draining out, but if it cannot drain, or too much is produced, then your eye pressure will rise.

Why can increased eye pressure be serious?

If the optic nerve comes under too much pressure then it can be damaged. How much damage there is will depend on how much pressure there is and how long it lasted, and whether there is a poor blood supply or other weakness of the optic nerve. A really high pressure will damage the optic nerve immediately. A lower level of pressure can cause damage more slowly, and then you would gradually lose your sight if is not treated. Damage to the optic nerve is indicated by the amount of ‘cupping’ of the optic nerve head. The cup is the paler central depression in the centre of the optic nerve head (also called the optic disc), and this progressively enlarges as more damage is done (figure 2).

The danger with glaucoma is that your eyes may seem perfectly normal. There is no pain and your eyesight will seem unchanged in the early stages, but your vision is being damaged. As the optic disc cupping progresses, so does the damage to your peripheral field of vision.

The early loss in the field of vision is usually filled in by your brain so you do not see a ‘black hole’ in your vision. Over time as the visual field loss progresses, you will start to become aware of blurred or blank areas in your vision. If untreated, these areas will spreads both outwards and inwards. The centre of the field is last affected so that eventually it becomes like looking through a long tube, so-called 'tunnel vision'. In time, even this sight would be lost (Figure 3).

Are there different types of glaucoma?

There are four main types of glaucoma.

Primary Open Angle Glaucoma (or chronic glaucoma) - this is the most common type of glaucoma. Aqueous can get to the angle (open angle) but the drainage channels slowly become blocked over many years . The eye pressure rises very slowly and there is no pain to show there is a problem, but the field of vision gradually becomes impaired.

Acute Angle Closure - (acute = sudden) is much less common. This happens when there is a sudden and more complete blockage to the flow of aqueous fluid in the eye. This is because a narrow 'angle' closes to prevent fluid getting to the drainage channels. Long-sighted eyes and Asian races are more at risk of this type of glaucoma. This can be very painful and will cause permanent damage to your sight if not treated promptly. More information is available on this under ‘Acute Glaucoma’.

Secondary Glaucoma - this less common type of glaucoma happens secondary to other conditions in your eye such as pseudo-exfoliation syndrome and uveitis (inflammation).

Neovascular Glaucoma - this type of glaucoma is rare and happens as a consequence of vascular problems within the eye such as diabetic retinopathy and retinal vein occlusions.

How is glaucoma detected?

Some form of glaucoma affects about 2 in 100 people over the age of 40. The incidence of glaucoma increases with age, so you should have eye tests at least every two years after the age of 40. Tests for glaucoma include:

Measuring the intraocular pressure (IOP).

Examination of the back of your eye to look at the optic disc for evidence of cupping.

Visual field test - this test is done with a machine that presents spots of light to your peripheral field of vision to determine if there are any damaged areas (areas where you do not see the spots).

Optical coherence tomography (OCT) - is a non-invasive imaging test that uses light waves to take cross-section pictures of your retina. It is able to accurately measure the thickness of the nerve layer around the optic nerve head, so is capable of detecting progressive damage even when the visual field test looks ok.

Your optometrist can screen you for glaucoma. Your optometrist will also be able to determine if you are at risk of Acute Angle Closure. If there is any concern that you may have glaucoma or are at risk of Acute Angle Closure, you should be referred to see an Ophthalmologist for assessment and treatment.

How is chronic glaucoma treated?

The aim of glaucoma treatment is to reduce the pressure in your eye in order to prevent or minimise further damage to your optic nerves. This is achieved by one or more of the following.

Medication - treatment to lower the pressure is usually started with eye drops. These act by reducing the amount of fluid produced in the eye or by opening up the drainage channels so that more fluid can drain away. Sometime more than one drop type is necessary to lower the pressure. It is very important that you use your eye drops regularly and correctly.

Selective Laser trabeculoplasty (SLT) - is often used to treat chronic glaucoma. The laser opens the clogged areas in the drainage angle and makes it easier for fluid to flow out of the eye.

Minimally-invasive glaucoma surgery (MIGS) - is a new and evolving area of glaucoma surgical treatment that is minimally invasive and so has a rapid recovery time. It has a high safety profile and low risk of complications. it involves placement of one or more ‘stents’ into the drainage angle of the eye to bypass the obstruction, allowing aqueous fluid to flow out of the eye. Currently, 3 devices are licensed in Australia for insertion at the time of cataract surgery. These are the iStent, the iStent inject, and the Hydrus Microstent. All devices reduce the number of medications on average by one, which for many patients will mean that they no longer require glaucoma drops. This treatment if only suitable for mild to moderate glaucoma.

Trabeculetomy or drainage tube - occasionally, an operation may be required to lower the pressure in your eye. This type of surgery creates an alternate outflow for the aqueous fluid in the eye to the subconjunctival space . This type of surgery carries a higher risk of complications than MIGS.

Your specialist will discuss with you which is the best method in your particular case. In many cases, a combination of treatments is required.

Can chronic glaucoma be cured?

Although damage already done cannot be repaired, with early diagnosis and careful regular observation and treatment, damage can usually be kept to a minimum, and good vision preserved.